In the construction of steel structures, such as bridges and buildings, it is common to join together two or more joint members (e.g., a beam to a column) by forming apertures in each of the joint members, forming matching apertures in splice plates spanning between the joint members, inserting bolts through the apertures, and securing the bolts on the opposite side of the joint members with nuts. Installation requirements of fasteners for these structural applications are detailed in a number of standards: Original Equipment Manufacturing standards, Department of Transportation construction standards, and other well-known industry standards such as those published by the Research Council on Structural Connections (RCSC). Other similar standards from different consensus bodies also exist in different markets.
The industry uses a number of different fasteners, including (1) hex-head and flange head fasteners; (2) pin- and collar fasteners; (3) and twist-off type fasteners. Each of these fasteners is summarized below. It is further known in the art to use a direct tension indicating (DTI) washer, also referred to as a load indicating washer or fastener, to control the tightening of a fastened joint by developing an indication of the tension in the fastener. It is still further known to include an indicating material, such as an extrudable silicone, with the DTI washer to provide a clear visual indication that a predetermined tension has been achieved in the fastener.
Hex-head fasteners can be installed using conventional air (pneumatic) impact wrenches and other wrenches including hand operated, electrically operated, and hydraulic wrenches. These fasteners are typically installed using either torque-based, compression-based, or degree-based installation techniques. Hex-head fasteners may have complicated washer usage requirements, depending on hole geometry, connection materials, and installation method. Installation of any hex-head fastener may require at least two installers to ensure proper nut rotation relative to the bolt. In some installations, the bolt may shift or “roll” in the hole during installation. For this reason many installation specifications require match marking of bolts and nuts. This operation is often inaccurate, subject to operator or inspector knowledge, and can even be replicated after installation on fasteners which have not been tensioned.
Pin-and-collar fasteners (also known as lock-pin and collar systems) operate on a principle of direct tension. Examples of pin-and-collar fasteners can be found at least in U.S. Pat. Nos. 2,531,048; 3,057,246; 3,915,053; 4,943,196; 5,049,016; and 5,562,379. Pin-and-collar fasteners have a pin with a first portion including a first plurality of annular rings (as opposed to the helical threads on the bolt of the twist-off fastener), a second portion with a second plurality of annular rings, and a neck portion between the first portion and the second portion. The first portion of the pin is connected to a head of the fastener, and the second portion of the pin defines an end of the fastener. A collar is disposed about the pin for engagement with the first portion. The collar is deformed with cold working about the pin and the first plurality of annular rings. The pin breaks off at the neck portion when the tensile capacity of the neck portion is exceeded. These fasteners are not removable and the tooling required to install these fasteners is expensive and cumbersome. Although these fasteners have a demonstrated capability to fracture in the neck, providing high assurance of direct tensile loads, they have a problem with properly snugging connections, particularly those involving large plies. At a certain point of deformation into the annular rings, the tensile loads begin to be applied against the rings on the pin, not directly to the head bearing surface on the opposite side of the connection. These fasteners are not able to perform efficient, sequential snugging operations.
Twist-off fasteners rely on torque-based installation. Examples of twist-off fasteners and related tooling are described in the industry standard specifications ASTM F1852 and F2280, and JSS S10T, and at least in GB 834787 A and in U.S. Pat. No. 2,928,302; 4,637,764; 4,659,267; 4,836,063; and 5,108,238. These fasteners are known by various other names, including TC bolts, Tru-Tension bolts, Tension Control bolts, Torque Control bolts, Tor-Shear bolts, and LeJeune bolts, among others. Twist-off fasteners have a bolt with a threaded shank and a nut in threaded engagement with the threaded shank. The bolt further has a head at one end of the shank and a splined end at the other end of the shank. Importantly, there is a shear groove between the splined end and the threaded shank, causing a stress concentration in that area. A manual wrench or power wrench, which has an inner socket engaged with the splined portion and an outer socket engaged with an outer surface of the nut, applies a torsional force on the bolt and nut. When the torsional load on the nut exceeds the shear capacity of the shear groove, the splined end is removed from the end of the bolt. In such fastener assemblies, the fastener tensions are related to calculated coefficients of friction, not the actual tensile or clamp load. Therefore, this fastening system is highly dependent on frictional engagement, and the effectiveness of the fastener (in other words, the ability of the fastener to achieve and maintain a desired clamp load) can vary with temperature, lubrication, weathering, and other environmental factors.
Regardless of the type of fastener used, a DTI washer can be combined with the fastener to control the tightening of a fastened joint by developing an indication of the tension in the fastener. Using DTI washers to measure tension in the fastener is preferable to measuring tension based on torque control because torque control measurements can result in wide variations in the tension in the fastener. Variations in the tension may result from factors such as variations in fastener lubricity, burrs on the thread of threaded fasteners, variations in the friction at the bearing surface between the joint member and the threaded fastener, dirt or corrosion on the threads, temperature, and the like.
A typical DTI washer includes a plurality of protuberances which are struck and partially sheared from an annular body to leave indentations in the annular body. For example, as depicted in FIGS. 1A, 1B, and 1C, a prior art DTI washer 10 includes an annular body 20 surrounding a central hole 22. FIG. 1A depicts a top view of the prior art DTI washer 10, FIG. 1B depicts a bottom view of the prior art DTI washer 10, and FIG. 1C depicts a cross-sectional view of the prior art DTI washer 10 along line 1C-1C of FIG. 1A. A first face 24 of the annular body 20 includes a plurality of protuberances 30 surrounding the central hole 22 along a concentric circle 32. A second face 26 of the annular body 20 includes a plurality of indentations 40. The area of the annular body 20 between the plurality of indentations 40 and the central hole 22 is referred to as the web 50. DTI washers of this type are disclosed in U.S. Pat. No. 5,015,132. Although many variations of these washers are known (for example, U.S. Pat. No. 5,667,346 discloses DTI washers having curved protuberances), prior art DTI washers share a common feature of the protuberances 30 being aligned with and centered over the indentations 40.
Referring to FIGS. 2A and 2B, the prior art DTI washer 10 may be used to join two or more joint members 110 in an exemplary joint assembly 100. As depicted in FIG. 2A, the prior art DTI washer 10 is placed on a first side of the joint members 110. An externally threaded member, such as a bolt 120 having a head 122 and a threaded shank 124, is then inserted through the joint members 110 and the central hole 22 of the prior art DTI washer 10 so that a bearing surface 126 of the bolt 120 contacts the protuberances 30 of the prior art DTI washer 10. Next, an internally threaded member, such as a nut 130, is attached to the shank 124 of the bolt 120 on a second side of the joint members 110 opposite the head 122 of the bolt 120. In some cases, a washer 140 may be positioned between the second side of the joint members 110 and the nut 130 to isolate the turning of the nut 130 from the joint members 110.
As depicted in FIG. 2B, as the nut 130 is rotated and tightened onto the threaded shank 124 of the bolt 120, the protuberances 30 are forced into the indentations 40. Tightening is stopped once the gap between the head 122 of the bolt 120 and the prior art DTI washer 10 disappears or is reduced to a desired distance which indicates the desired amount of tension in the threaded shank 124. The dimensions and material properties of the prior art DTI washer 10 determine how much tension is required to force the protuberances 30 back into the indentations 40. The indentations 40 may be at least partially filled as a result of the protuberances 30 being forced back into the annular body 20, and perform no useful function in the operation of the prior art DTI washer 10. Rather, the indentations 40 are a result of the displacement of material during the forming operation which creates the protuberances 30.
Regardless of the embodiment, the DTI washer may include an indicating material to provide a clear visual indication that a predetermined tension has been achieved in the fastener. U.S. Pat. No. 5,931,618 titled “Direct Tension Indicating Washers” describes such a material, and the DTI washer with the material is sold commercially under the registered trademark Squirter®. The DTI washer has a first surface with at least one protuberance formed on that first surface and a second surface with at least one indentation formed on the second surface opposite the protuberance. The inner diameter of the DTI washer has an inner diameter wall meeting the second surface at an inner diameter edge. The outer diameter has an outer diameter wall meeting the second surface at an outer diameter edge. The inner diameter edge and the outer diameter edge lie in different planes. This difference causes the flattened DTI washer to exert force against a bolt and maintain bolt tension. The DTI washer includes indicating material positioned in the indentation and a channel formed in the second surface leading from the indentation to the outside diameter of the DTI washer.
U.S. Pat. No. 6,425,718 titled “Direct Multi-Tension Indicating Washer Having Bumps of a First and Second Height” discloses an alternative to the DTI washer disclosed in U.S. Pat. No. 5,931,618. The alternative DTI washer extrudes indicating material of varying colors depending on the compression force applied to it by providing a first and second surface having respective protuberances (i.e., bumps) and indentations. The compression force on the washer is equal to the tension of a bolt passing through the washer. The indentations in the second surface are filled with a solid extrudable colored material. By making at least a first protuberance taller than at least a second protuberance, the first protuberance will compress before the second protuberance, causing it to extrude the color material in its corresponding indentation before the second protuberance does the same as the tension in the bolt is increased. By making the material under the first protuberance a different color than the material under the second protuberance, an operator can determine which of two desired tensions the bolt has reached. In use, an operator merely tightens the bolt passing through the washer or a nut threaded to the bolt until the bolt reaches a first desired tension as indicated by the extruded material of the first color, such as green. An intervening step is performed at the first bolt tension, if required. Then the operator continues to tighten the bolt or nut until the bolt reaches a second desired tension as indicated by the extruded material of the second color, such as red.
Each of the existing different fasteners and DTI washers has shortcomings. When twist-off type tension control bolts are fully tightened, for example, their tips shear off to give an indication that final full preload has been achieved. Thus, if only snug tightening is required, the load is presumed adequate. These types of bolts do not offer any indication of snug before final full tensioning. Fastener systems that use Squirter®-type DTI washers also do not provide a visual indication of snug. Further, these systems do not have a distinct two-stage flattening of the protuberances that allows a check for both snug and final full tightening. In fact, there is no known prior art system that includes a load-indicating function for both the snug stage and the final tightening stage.
There is also no known DTI washer with protuberances of different heights to perform different functions, or unique shapes to encapsulate an elastomeric material to be emitted at the right time indicating a properly snugged or fully tightened bolt. Prior art bolting practices, such as use of a calibrated wrench or turn-of-nut, require continuous inspection of tightening operations so that an operator can verify that an initial tightening operation is being performed. This inspection is an on-going activity, as there is no evidence following the snugging that the work has been performed. The inspection is a verification or confirmation that performance of this work is being observed.
To overcome the shortcomings of existing DTI washers, an object is to provide an improved DTI washer. Another object is to enable a DTI washer to perform two or more discreet load measurements accurately. A related object is to enable a DTI washer to perform two or more different operations as a function of the plastic deformation of the protuberances.
It is still another object to use uniquely shaped and varying height protuberances to enable a visual indication that the fastener assembly is adequately tightened. A related object is to overcome the limitations of the existing DTI washers, especially those marketed as Squirter® DTI washers, which suffer from the elastomeric indicating material giving premature visual indications that a bolt is adequately tightened. Another related object is to overcome geometric limitations of standard DTI washers by introducing protuberance shapes that protect the elastomeric indicating material during handling, packaging, shipping, and warehousing so that the elastomeric material can visually indicate when adequate bolt tension has been achieved.
An additional object is to delay, through geometric function, the emission of an elastomeric material until the fastener assembly is adequately tightened. Yet another object is to capture the elastomeric material so that during use it is against a relatively smooth surface (e.g., a washer or bolt) that ensures predictable and repeatable accurate results. (Contrast such a configuration with prior art squirt-type devices, for which the elastomeric material is placed against the unpredictable surface of structural steel which may vary from very smooth to rough.) It is still a further object to provide visual proof that the “snug” tight condition has been achieved for a fastener and that the fastener is in firm contact with a component to be joined.